Understanding Correlation Between CO2 Insertion Mechanism and Chain Length of Diamine in Metal-Organic Framework Adsorbents.

Although CO2 insertion is a predominant phenomenon in diamine-functionalized Mg2 (dobpdc) (dobpdc4- =4,4-dioxidobiphenyl-3,3'-dicarboxylate) adsorbents, a high-performance metal-organic framework for capturing CO2 , the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg2 (dobpdc) systems with open metal sites grafted by primary diamines NH2 -(CH2 )n -NH2 were developed, with en (n=2), pn (n=3), bn (n=4), pen (n=5), hn (n=6), and on (n=8). Based on CO2 adsorption and IR results, CO2 insertion is involved in frameworks with n=2 and 3 but not in systems with n≥5. According to NMR data, bn-appended Mg2 (dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO2 insertion, as validated by first-principles density functional theory (DFT) calculations. For 1-hn and 1-on, DFT calculations indicated that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc4- . Computed CO2 binding enthalpies for CO2 insertion (-27.8 kJ mol-1 for 1-hn and -20.2 kJ mol-1 for 1-on) were comparable to those for CO2 physisorption (-19.3 kJ mol-1 for 1-hn and -20.8 kJ mol-1 for 1-on). This suggests that CO2 insertion is likely to compete with CO2 physisorption on diamines of the framework when n≥5.